Date: Fri, 24 Jul 2015 22:51:19 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Willy Tarreau <w@1wt.eu>, Steven Rostedt <rostedt@goodmis.org>,
        Andy Lutomirski <luto@amacapital.net>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Borislav Petkov <bp@alien8.de>, Thomas Gleixner <tglx@linutronix.de>,
        Brian Gerst <brgerst@gmail.com>
Subject: Re: Dealing with the NMI mess
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On Fri, Jul 24, 2015 at 01:22:11PM -0700, Linus Torvalds wrote:
> On Fri, Jul 24, 2015 at 12:55 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > I worry that we'll end up running the do_debug() handlers from effective
> > NMI context.
> >
> > The NMI might have preempted locks which these handlers require etc..
> 
> If #DB takes any locks like that, then #DB is broken.
> 
> Pretty much by definition, a data breakpoint can happen on pretty much
> absolutely any code. This is in no way NMI-specific as far as I can
> tell.
> 
> Do we really take locks in the #DB handler?

do_debug()
  send_sigtrap()
    force_sig_info()
      spin_lock_irqsave()

Now, I don't pretend to understand the condition before send_sigtrap(),
so it _might_ be ok, but it sure as heck could do with a comment.
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